The present invention relates to a device for generating a free cold plasma beam.
In surface treatment of workpieces the use of plasma technology is well known. There are thermal plasmas generated at atmospheric pressure or in a near-atmospheric pressure range and at temperatures that may reach thousands of degrees. There are also cold plasmas used in vacuum. They provide high activation energies for chemical reactions, typically do not heat the gas over 500xc2x0 C. and thus allow surface treatements and layer depositions on temperature-sensitive workpieces. In addition, many layers having technically relevant properties can only be synthesized using cold plasmas. In particular, plasmas produced by microwave radiation offer high deposition rates for vacuum coating processes, as well as a good layer quality. Typical plasma densities for this method are approximately 1011 ions/cm3.
However, in using these technologies, it is often difficult to spatially separate plasma generation from its application for surface treatment or layer deposition; otherwise the equipment for generating the plasma within the equipment is undesirably also affected, for example, by deposition processes, which results in high maintenance and downtime costs.
Therefore it is known from the related art that a free, however hot, i.e. thermal, plasma beam can be emitted from a plasma source and thus plasma production and industrial use of this plasma beam, for example, in a downstream coating apparatus, can be spatially separated. A free plasma beam is defined as a plasma beam that propagates in a larger volume without being guided by equipment or auxiliary devices. Such beams include different versions of plasma sprays such as described in the publication xe2x80x9cVI. Workshop fxc3xcr Plasmatechnik (1998), TU Ilmenau. FG Plasma- und Oberflxc3xa4chentechnik.xe2x80x9d These free, but hot, or thermal, plasma beams do not widen substantially as they enter in the coating apparatus which is under atmospheric or near-atmospheric pressure and can thus be accurately guided to the point of use.
Many industrially relevant surface treatment and coating processes can only be carried out in vacuum with cold plasmas for reasons of process technology. However, in vacuum, obtaining a free bundled beam that is not disturbed by diffusion or convection is problematic.
Also known from the related art is the propagation of plasmas within a tube that is transparent to electromagnetic radiation due to electromagnetic surface waves whose frequency is in the microwave range or in near-microwave frequency bands. These surface waves propagate along the boundary surface between the electrically conductive plasma and the tube. Plasma sources based on this principle are referred to as xe2x80x9cwave launchersxe2x80x9d (M. Moisan and Z. Zakrewski, Plasma sources based on the propagation of electromagnetic surface waves, J. Phys. D: Appl. Phys. 24 (1991) 1025-1048) and work both in vacuum and under atmospheric pressure. At atmospheric pressure this plasma may propagate outside the tube for a few centimeters as a thermal plasma in the form of a free, bundled beam. To date, this has not been considered possible for cold plasmas and, in particular, for cold microwave plasmas or high-frequency plasmas in vacuum (J. Margot and M. Moisan, Physics of surface wave discharges, Proceedings of Plasma Processing of Semiconductors, Chateau de Bonas, France (1997), 187-210).
A plasma is typically densest at the point of injection, i.e., directly on or in the plasma source. Therefore, in the related art, distinction is made between direct processes and remote processes. In direct processes deposition takes place by adding reactive gas directly at the point of injection, i.e. at the point of highest plasma density, so that in this case the excitation states can be used most effectively for layer deposition. One disadvantage of the direct processes is, however, that the injection structures are necessarily involved, due to undesirable deposition processes, in the coating operation. In order to avoid this disadvantage, remote processes are often used. In such processes the reactive gas is only added to a plasma at some distance from the injection structure in order to reduce contamination. However this also reduces plasma density and therefore effectiveness.
Production of a free, cold, high-frequency plasma beam and a suitable apparatus, where initially a cold plasma is produced in a hollow body transparent to electromagnetic radiation, this plasma then being guided into a work chamber under vacuum, separate from the plasma source and from the production of this beam, where it propagates over a longer distance of up to 1 m, is not yet known.
The device according to the present invention having the characterizing features of claim 1 has the advantage over the related art that the cold gas plasma in the form of a bundled high-frequency plasma beam is conducted into a work chamber under vacuum that is separate from the plasma source and from the production of this beam, and there propagates as a free, cold plasma beam having an almost constant diameter, i.e., with only a slight diffusion widening. This allows processes that were previously impossible, in particular in industry, such as, for example, specific treatments of workpieces, to be carried out. The free, cold plasma beam is stable, due to the design of the work chamber and plasma producing zone and their connection without any additionally generated electromagnetic fields, over a larger distance in the work chamber and remains bundled. This also allows geometrically complex workpieces to be processed.
Furthermore, the device according to the present invention offers the possibility of combining the advantages of direct coating processes and remote processes. The production of a free plasma beam makes direct coating in an intensive plasma beam possible by adding a reactive gas while the coating process is farther removed from the point of plasma production. This considerably simplifies maintenance and reduces system downtime, while increasing the effectiveness of the coating process.
The cold plasma beam according to the present invention also has a very high plasma density of more than 1012 ions/cm3, which results in high effectiveness. Thus high growth rates are achieved in coating processes and high removal rates in etching processes.
Because the cold plasma beam only slightly heats the gas while making a high activation energy available for chemical reactions, temperature-sensitive workpieces can also be treated.
Thus, it is very advantageous that the free, cold, high-frequency plasma beam can be used in a work chamber under vacuum, in particular a vacuum from 0.1 Pa to 1000 Pa. On the one hand, the device according to the present invention can be operated in a chamber that is not under high vacuum; on the other hand, vacuum is used to effectively carry out many technically relevant plasma processes. Examples include etching processes, layer adhesion enhancing processes, and deposition of extremely compact layers having a low degree of surface roughness.
The diameter and length of the free plasma beam and thus variations in space in the deposition, growth, and etching rates can be controlled in a simple manner via process parameters such as, for example, gas pressure, gas flow rate, microwave power or high frequency power, as well as the high frequency or microwave frequency used to generate the gas plasma. The gas plasma is advantageously generated within the hollow body that is transparent to electromagnetic radiation via an essentially known commercially available microwave plasma source having slot antenna injection. As an alternative, a high-frequency plasma source, which is also commercially available, can also be used.
In addition, it is highly advantageous that the direction of propagation of the free plasma beam in the work chamber is not, or is only slightly, influenced by the direction of the gas flow within the work chamber, since the electromagnetic field distribution has primary importance in determining the direction of propagation. Therefore, the gas may flow in the work chamber perpendicularly to the direction of propagation of the free plasma beam, for example, without the plasma beam becoming noticeably curved. This property allows two intersecting plasma beams, directed against one another, for example, along a single line, to be generated in the work chamber. The resulting combined beam is then characterized by increased axial homogeneity. This arrangement is particularly well-suited for coating large-surface workpieces.
Furthermore, the injection geometry, i.e., the shape of the hollow body transparent to electromagnetic radiation in which the plasma is initially generated and the beam outlet opening can be adjusted to specific requirements of the beam geometry in the work chamber. Thus, the flat design of the hollow body, for example, as a parallelepiped having a beam outlet opening in the form of a slot, results in the generated free plasma beam entering the work chamber as a flat plasma beam. The free plasma beam thus generated is advantageously well-suited for coating, cleaning, or etching of large-surface workpieces.
The free plasma beam can be used in combination with other processes, including processes that do not involve a plasma beam source, in a simple manner.